The invention relates to high speed continuous draw tape plastic bag manufacture and, in particular, to a cam driven reciprocating thermoplastic film heat sealing apparatus.
U.S. patent application Ser. No. 652,254, now U.S. Pat. No. 4,624,654, assigned to the assignee of this invention, describes a method and apparatus for manufacturing plastic film draw tape bags. That application and all related applications disclosed therein are incorporated by reference herein.
The aforesaid U.S. patent application discloses a method of and apparatus for manufacturing draw tape bags in which a medianly folded, continuous length of plastic film is continuously advanced from a feed roll through a hemming device for inwardly turning the adjoining free edges of the film, a punch for punching openings through the hemmed edges of the film at regular intervals, and a device for inserting continuous plastic draw tape strips into each hem of the continuously moving web. The web with draw tape strips is then fed through a reciprocating heat sealer for sealing the hems and a reciprocating transverse sealing and cutting device for sealing and cutting individual bag bodies from the continuous length of film. A dancer between the draw tape strip feeding device and the heat sealer accumulates the continuous fed film web and draw tape strips for intermittent advances of the web and strips through the downstream portion of the line.
In one type of known reciprocating bar sealer, a pair of opposed elongated heated bars are positioned one above the other on either side of a platen, to receive a pair of lengths of thermoplastic film therebetween. The bars are supported, moved and guided solely by a pair of air cylinders, each coupled to the middle of a separate one of the bars. This construction results in two major types of problems. First, because of the loads to which they are subjected, the air cylinders quickly wear and must be regularly replaced. In continuous production, air cylinders may require replacement as frequently as once per day. Second, it is difficult if not virtually impossible to obtain alignment of the bars in contact with the plastic sheet. This results in skips or gaps and unevenness in the heat seal. This is true even where the platen is covered with a cushioning material such as a silicon rubber layer.
Another type of known prior art reciprocating bar sealer employs a single air cylinder to reciprocate both bars along guides by linkages. The seal bars are rigidly mounted with the air cylinder through the linkages. Some slight improvement is had.
It would be useful to provide a reciprocating bar heat sealer automatically compensating for alignment as well as slight machining errors.
It would be desirable to improve the effective cycling rate of reciprocating heated bar sealers for higher manufacturing rates.